Re: [PATCH v3] mm,oom: speed up select_bad_process() loop.

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 18 May 2016 16:15:45 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> > This patch adds a counter to signal_struct for tracking how many
> > TIF_MEMDIE threads are in a given thread group, and check it at
> > oom_scan_process_thread() so that select_bad_process() can use
> > for_each_process() rather than for_each_process_thread().
> 
> OK, this looks correct. Strictly speaking the patch is missing any note
> on _why_ this is needed or an improvement. I would add something like
> the following:
> "
> Although the original code was correct it was quite inefficient because
> each thread group was scanned num_threads times which can be a lot
> especially with processes with many threads. Even though the OOM is
> extremely cold path it is always good to be as effective as possible
> when we are inside rcu_read_lock() - aka unpreemptible context.
> "

This sounds quite rubbery to me.  Lots of code calls
for_each_process_thread() and presumably that isn't causing problems. 
We're bloating up the signal_struct to solve some problem on a
rarely-called slowpath with no evidence that there is actually a
problem to be solved.

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